Schottky diode having increased active surface area with improved reverse bias characteristics and method of fabrication

ABSTRACT

A Schottky diode comprises a semiconductor body of one conductivity type, the semiconductor body having a grooved surface, a metal layer on the grooved surface and forming a Schottky junction with the semiconductor body. The semiconductor body preferably includes a silicon substrate with the grooved surface being on a device region defined by a guard ring of a conductivity type opposite to the conductivity type of the semiconductor body, and a plurality of doped regions at the bottom of grooves and forming P-N junctions with the semiconductor body. The P-N junctions of the doped regions form carrier depletion regions across and spaced from the grooves to increase the reverse bias breakdown voltage and reduce the reverse bias leakage current.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/283,537 filed Apr. 1, 1999 for “Power Rectifier Device” U.S. Pat. No6,331,445 and pending application Ser. No. 09/620,074 filed Jul. 20,2000 for “Schottky Diode Having Increased Active Surface Area WithImproved Reverse Bias Characteristics And Method Of Fabrication”, thedescriptions of which are incorporated herein by reference. And a CIP of09/544,730 filed Apr. 6, 2000.

BACKGROUND OF THE INVENTION

This invention relates generally to power semiconductor devices, andmore particularly the invention relates to a Schottky diode device and amethod of making same.

Power semiconductor rectifiers have a variety of applications includinguse in power supplies and power converters. Heretofore, Schottky diodeshave been used in these applications. A Schottky diode is characterizedby a low turn-on voltage, fast turnoff, and nonconductance when thediode is reverse biased. To create a Schottky diode a metal-siliconbarrier must be formed. In order to obtain the proper characteristicsfor the Schottky diode, the barrier metal is likely different than themetal used in other process steps such as metal ohmic contacts.

Copending application Ser. No. 09/283,537, supra, discloses a verticalsemiconductor power rectifier device which employs a large number ofparallel connected cells, each comprising a MOSFET structure with agate-to-drain short via common metalization and an associated Schottkydiode. This provides a low Vf path through the channel regions of theMOSFET cells to the source region on the other side of the device. Themethod of manufacturing the rectifier device provides highly repeatabledevice characteristics at reduced manufacturing costs.

Copending application Ser. No. 09/620,074 effectively increases diodesurface by providing a trenched surface on which Schottky material isdeposited. The resulting structure has increased current capacity forsemiconductor chip area. In accordance with the method of fabricatingthe Schottky diode, a guard ring is formed around a device region in asemiconductor chip surface. The guard ring has conductivity typeopposite to that of the chip body. Using photoresist masking andetching, a plurality of trenches are etched in the surface of the deviceregion, thereby effectively increasing the active surface area in thedevice region. A Schottky metal is then deposited over the device regionin the trenches, and electrode material is deposited to form top andbottom electrodes for the Schottky diode.

The present invention is directed to an improved method of manufacturinga Schottky rectifier device and the resulting structure.

SUMMARY OF THE INVENTION

In accordance with the invention, the effective surface area of aSchottky diode is increased by providing a trenched surface on whichSchottky material is deposited. The resulting structure has increasedcurrent capacity for semiconductor chip area. To provide for higherreverse breakdown voltage and lower reverse leakage current, a P-Njunction is formed at or near the bottom of the trench surfaces so thatwhen the Schottky diode is reversed biased, a charge depletion regionspreads across and spaced from the trench surface, thereby increasingthe reverse breakdown voltage and reducing reverse leakage current.

In accordance with a preferred embodiment, the PN junction is formed byion implantation in alignment with the trench walls. A photoresist maskcan be employed to define. the ion implanted surface area.Alternatively, an oxide layer can be selectively formed on the surfacesof the trench surface to limit the implantation of ions.

The invention and objects and features thereof will be more readilyapparent from the following detailed description and dependent claimswhen taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-10 are section views illustrating steps in fabricating aSchottky diode in accordance with one embodiment of the invention.

FIG. 11 is a plan view of the resulting Schottky diode using theprocesses of FIGS. 1-10.

FIGS. 12-18 are section views illustrating steps in fabricating aSchottky diode in accordance with another embodiment of the invention.

Like elements in the figures have the same reference numerals.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

FIGS. 1-10 are section views illustrating process steps in fabricating aSchottky diode in accordance with one embodiment of the presentinvention. In FIG. 1 an N+ silicon substrate 10 has an N− epitaxialsilicon layer 12 thereon with layer 12 having a conductivity of about0.1-10 ohm cm. A field oxide 14 is grown or deposited on the surface ofepitaxial layer 12 to a thickness of 300-1000 nm.

FIG. 2 a photoresist pattern 16 is formed over field oxide 14 withopenings through the photoresist at 18 for the removal of exposed fieldoxide and implantation of boron or other P-type dopant for use infabricating a guard ring around a device region. Boron is implanted at20 at a dose of about 10¹¹-10¹⁴/cm². The boron implant can be afterphotoresist removal.

In FIG. 3 the photoresist 16 is removed and the body is heated for borondrive-in in forming deep P-regions 22 which are the guard ringsurrounding a device region under field oxide 14. An additional boron(BF₂) implant is made for high surface concentration to form good ohmiccontacts to the guard ring. Rapid thermal annealing is employed toactivate the BF₂ dopant.

In FIG. 4 a second photoresist mask pattern 24 is used to expose theactive device region so that the overlying field oxide 14 can be laterremoved, as shown in FIG. 5 in which a suitable oxide etchant such as HFsolution is employed.

Thereafter, in FIG. 6 a third photoresist pattern 28 is formed over thesurface of the active region to define trenches in the surface of theactive region by etching as illustrated in FIG. 7. A plasma or reactiveion etch can be employed for etching the trenches with either verticalor sloped sidewalls. Thereafter, boron or BF₂ ions are implanted in thebottom of the trenches in surface areas 30 of epitaxial layer 12 usingphotoresist pattern 28 as an ion implant mask.

Photoresist pattern 28 is then removed and rapid thermal annealing isemployed to form P-regions 30 at the trench bottoms, thereby forming P-Njunctions with epitaxial layer 12. A Schottky metal 32 is then depositedover the surface of the active region in the trenches as shown in FIG.8. The Schottky metal may comprise molybdenum, aluminum, platinum orother known material for forming Schottky junctions with silicon such asrefractory metals and silicides thereof. Finally, a bottom electrode 34and a top electrode 36 are deposited for making contact to the finishedSchottky diode. The electrode material can be titanium, titaniumnitride, nickel, silver, gold, copper, or other suitable material andcombinations thereof.

FIG. 9 illustrates the finished device with a forward bias and depletionregions 40 around P-regions 30 and the PN-junction formed with N−epitaxial layer 12. With a forward bias, current flows from the topelectrode to the bottom electrode as. illustrated at 42.

FIG. 10 illustrates the finished device with a reverse bias applied tothe top of electrode 36 and bottom electrode 34, whereby current ceasesto flow. The reverse bias on the PN-junction formed by P-regions 30, andthe N− epitaxial layer 12 expands across and spaced from the Schottkydiode as shown at 40. The increased charge depletion region with thereverse bias of the electrodes increases the reverse breakdown voltagefor the device and lowers reverse leakage current.

FIG. 11 is a plan view illustrating the top of the completed Schottkydiode. The trenched surface in the active region surrounded by guardring 22 increases the surface area of the Schottky diode and thisincreases current density and current carrying capacity for unit surfacearea of the semiconductor device.

As shown in FIG. 7, some of the boron or BF₂ can be implanted into thesidewalls of the trenches, especially when the sidewalls are sloped. Toprevent this an alternative embodiment is provided.

FIGS. 12-18 are section views illustrating the alternative embodiment ofthe invention. In this process, the steps illustrated in FIGS. 1-5remain the same. However, in FIG. 12, an insulator layer 50 having athickness on the order of 30-300 nm is grown or deposited on the surfaceof epitaxial layer 12. The insulator is preferably silicon oxide or asilicon nitride layer. Thereafter, photoresist pattern 28 is againformed over the surface of the active region to define trenches in thesurface of the active region by etching as illustrated in FIG. 13.Again, a plasma or reactive ion etch can be employed to form thetrenches with either vertical or sloped sidewalls. Insulator 50 isremoved prior to trench formation using either a wet or dry etch.

Next, as shown in FIG. 14, the photoresist 28 is removed, and anotherinsulator layer 52 is deposited over the surface of the device. Theinsulator can be silicon oxide or silicon nitride and is preferably inthe range of 30 to 300 nm in thickness.

Next, as shown in FIG. 15, anisotropic etch is used to remove theinsulator 52 from the bottom of the trenches but leaves insulator on thesidewalls and on the top of the trench surfaces, and boron or BF₂ isthen implanted into the bottom of the trenches in surface areas 30 ofepitaxial layer 12. This is similar to the process step of FIG. 7. Thesidewall spacer also reduces the bottom trench area, thereby reducingthe PN-junction area and increasing the Schottky conducting area.Thereafter, as shown in FIG. 16, rapid thermal annealing is employed toform P-regions 30 at the trench bottoms, and then the insulator 50, 52is removed from the walls of the trench. Schottky metal 32 is thendeposited over the surface of the trenched surface, similar to theprocess step of FIG. 8, followed by forming of bottom electrode 34 andtop electrode 36. Use of the oxide ion mask, as shown in FIG. 14.provides added protection against ion implantation into the sidewallsand top surfaces of the trenched surface.

Finally, the finished device is shown forward biased in FIG. 1 andreverse biased in FIG. 18, similar to FIGS. 9 and 10, above.

There has been shown an improved Schottky diode having increased currentcapacity for semiconductor chip area. The inclusion of PN-junctions atthe bottoms of the trenched surface permits the use of enhanceddepletion regions when the diode is reversed biased, thereby increasingthe reverse bias breakdown voltage, and reducing reverse bias leakagecurrent. While the invention has been described with reference tospecific embodiments, the description is illustrative of the inventionand is not to be construed as limiting the invention. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A Schottky diode comprising: a) a semiconductorbody of one conductivity type and having a major surface, b) a guardring of opposite conductivity type formed in the major surface of secondconductivity type and surrounding a device region, c) a plurality oftrenches in the major surface within the device region, d) doped regionsof said opposite conductivity type formed in the semiconductor body atthe bottom of trenches, said doped region forming P-N junctions with thesemiconductor body, and e) a metal overlying the device region and inthe plurality of trenches forming a Schottky junction with all topsurfaces and side walls of the trenches in the semiconductor body. 2.The semiconductor body of claim 1 wherein the semiconductor body issilicon.
 3. The semiconductor body of claim 2 wherein the semiconductorbody comprises a substrate and an epitaxial layer, the epitaxial layerbeing of N-conductivity.
 4. A Schottky diode comprising: a) a siliconsemiconductor body of one conductivity type and having a major surface,said silicon semiconductor body comprising a substrate and an epitaxiallayer, the epitaxial layer being of N-conductivity; b) a guard ring ofP-type conductivity type formed in the major surface of secondconductivity type and surrounding a device region, c) a plurality oftrenches in the major surface within the device region, d) doped regionsof said opposite conductivity type formed in the semiconductor body atthe bottom of trenches, said doped region forming P-N junctions with thesemiconductor body, and e) a metal overlying the device region and inthe plurality of trenches forming a Schottky junction with thesemiconductor body.
 5. The semiconductor body of claim 4 wherein themetal overlying the device region is selected from the group consistingof molybdenum, platinum, aluminum, refractory metal and suicidesthereof.
 6. The semiconductor body of claim 5 and further includingcontact metal on the bottom of the semiconductor body and on the metaloverlying the device region.
 7. The semiconductor body of claim 6wherein the contact metal is selected from the group consisting of Ti,TiN, Ni, Ag, Au, Cu, and combinations thereof.
 8. The semiconductor bodyof claim 4 and further including contact metal on the bottom of thesemiconductor body and on the metal overlying the device region.
 9. Thesemiconductor body of claim 8 wherein the contact metal is selected fromthe group consisting of Ti, TiN, Ni, Ag, Au, Cu, and combinationsthereof.
 10. A Schottky diode comprising a semiconductor body of oneconductivity type, the semiconductor body having a grooved surface, ametal layer on the grooved surface and forming a Schottky junction withall top surfaces and side walls of the grooved surface of thesemiconductor body, the grooved surface being on a device region of thesemiconductor body defined by a guard ring of a second conductivity typesurrounding the device region; and a plurality of doped regions ofopposite conductivity type in the semiconductor body at the bottom oftrenches, the doped regions forming P-N junctions with the semiconductorbody.
 11. The Schottky diode as defined by claim 10 wherein thesemiconductor body comprises a silicon substrate and an epitaxialsilicon layer of N-conductivity on the substrate, the epitaxial layerhaving the grooved surface.
 12. The Schottky diode as defined by claim11 wherein the metal layer is selected from the group consisting ofmolybdenum, platinum, aluminum, refracting metal, silicides thereof, andcombinations thereof.
 13. The semiconductor body for claim 3 wherein theguard ring is P-type conductivity.
 14. The semiconductor body of claim13 wherein the metal overlying the device region is selected from thegroup consisting of molybdenum, platinum, aluminum, refractory metal andsilicides thereof.
 15. The semiconductor body of claim 14 and furtherincluding contact metal on the bottom of the semiconductor body and onthe metal overlying the device region.
 16. The semiconductor body ofclaim 15 wherein the contact metal is selected from the group consistingof Ti, TiN, Ni, Ag, Au, Cu, and combinations thereof.
 17. Thesemiconductor body of claim 1 and further including contact metal on thebottom of the semiconductor body and on the metal overlying the deviceregion.
 18. The semiconductor body of claim 17 wherein the contact metalis selected from the group consisting of Ti, TiN, Ni, Ag, Au, Cu, andcombinations thereof.